1. Field of the Invention
This invention relates to a gate power supply circuit, and more particularly to a gate power supply circuit that supplies gate power to the gate drive circuit for a self-turn-off device from a main circuit, utilizing the switching action of a self-turn-off device etc.
2. Description of the Related Art
By employing self-turn-off devices in a power converter such as an inverter, the benefits are obtained of better suppression of source side and load side harmonics than hitherto, improvement of the power source power factor, and device miniaturization, etc. Hitherto, it was impossible to obtain self-turn-off devices suited for high voltage and larger current use. Recently however, it has become possible to manufacture self-turn-off devices typified by GTOs suited for high voltage and large current use. Application of self-turn-off devices to the high power field has therefore become common.
When applying self-turn-off devices such as GTOs to high voltage applications, the problem of the drive power source of the gate drive circuit of the self-turn-off device cannot be neglected. This problem is particularly severe in the case of GTOs which are switching devices mainly employed in the high power field. The reason for this is that a GTO is a current-controlled device, yet the degree of current amplification on turn-off is small, so, on turn-off, a very large current the amplitude of which is about one third through one fifth of that of the main circuit current, must be supplied to the gate of the GTO. Furthermore, even when the GTO is ON, current to the gate must be continued in order to reduce conduction loss. The power consumption of the gate drive circuit of a GTO is therefore at least 100 W per GTO, although this does vary depending on the type of GTO.
Since the gate drive circuit of a GTO is directly connected to the cathode and gate of the GTO that is being driven, it is electrically at the same potential as the cathode of the GTO that is being driven. When GTOs are connected in series, the gate drive circuit of each GTO is at a respectively different potential, so the power sources of the respective gate drive circuits must be at respectively different potentials. This means that the power sources of the gate drive circuits of the GTOs must be mutually insulated for each respective GTO.
An example of a conventionally used GTO gate power supply circuit is shown in FIG. 12. FIG. 12 shows a single GTO of a plurality of GTOs constituting a power converter and the associated snubber circuit and gate drive circuit.
In FIG. 12, a GTO 1 is the main switching device. A snubber diode 2 and a snubber capacitor 3 constitute a snubber circuit for suppressing the rate of voltage rise when GTO 1 is turned OFF, and suppress GTO loss on turn-off. A diode 4 is a free-wheeling diode that provides a current path for the main circuit current in regeneration mode. A resistor 5 is a DC balance resistor that performs the action of balancing the DC voltages apportioned to each GTO when a large number of GTOs are connected in series, so that they are not affected by the slight variations of the characteristics of each GTO.
The gate of GTO 1 is driven by a gate drive circuit 10. The ON/OFF signal of the gate is transmitted as an optical signal by an optical fiber 11 and is converted to an electrical signal by a photoreceptor module, not shown, in gate drive circuit 10. Since an optical signal is employed, the ON/OFF gate signals are automatically insulated for each GTO 1. Regarding the power source of gate drive circuit 10, this can be obtained by using a rectifier 12 to produce DC power by rectifying the high frequency (such as 20 kHz) AC power supplied from a high frequency AC power source 14 through an isolating transformer 13. High frequency AC power source 14 is located in a low potential zone and is supplied in common to each GTO; the difference in potential between GTOs 1 is insulated by means of isolating transformer 13 corresponding to each respective GTO 1.
However, there are considerable problems in applying the prior art to power converters in which a large number of GTOs are connected in series and the DC bus voltage exceeds a few tens of kV. This is because a large number of isolating transformers capable of withstanding a high voltage of a few tens of kV to be isolated across their poles and capable of insulating the high voltage are required in order to achieve delivery of high frequency AC power through isolating transformers 13 from the low potential zone. Not only do such isolating transformers require a large amount of space, but in addition they are enormously expensive. There were therefore problems on grounds of space and cost in applying the prior art to high voltage application, such as power converters, in which a large number of GTOs were connected in series.